Field of the Invention
This invention relates to an optical member that is required to show anti-reflection properties so as to effectively be used in an optical instrument such as a camera, a binocular, a microscope, or a projector, which optical member may typically be a lens, a meniscus lens in particular. The present invention also relates to an optical system using such an optical member and to a method of manufacturing such an optical member.
Description of the Related Art
Optical members such as lenses, prisms and so on to be used in optical instruments, including cameras, binoculars, microscopes, and projectors, are provided with anti-reflection means for preventing so-called harmful rays, which are rays of light other than an imaging light flux, from striking the image plane of the instrument. Such harmful rays include rays of light reflected from the light input/output surface (the surface at which light is to be transmitted without being reflected and which will be referred to as “optically effective area” hereinafter) of the optical member and those reflected from the edge portions of the end facets of the optical member and so on (the surfaces at which light is to be absorbed without being transmitted or reflected and which will be referred to as “optically non-effective area” hereinafter). Roughly, there are two types of techniques that are being employed to suppress harmful rays as listed below.                (1) Techniques for reducing reflections by improving the transmittance of light at the optically effective area        (2) Techniques for reducing reflections by improving the absorptivity of light at the optically non-effective area        
As a technique that belongs to type (1) above, a technique of forming an anti-reflection film on the optically effective area of an optical member by laminating dielectric thin films typically by means of sputtering or vacuum deposition has been and is being widely utilized. There are also known techniques of forming an anti-reflection film by forming a so-called textured structure (SWS), sub-wavelength being wavelengths not greater than the use wavelength of light to be used in operation, so as to alleviate the change in the refractive index of light of the use wavelength that can occur at the interface between the inside and the outside of the substrate of the optical member (see, inter alia, Japanese Patent Application Laid-Open No. 2006-259711 and Japanese Patent Application Laid-Open No. 2005-275372).
As a technique that belongs to type (2) above, a technique of applying a paint that is opaque relative to the use wavelength of light to the optically non-effective area of an optical member, which may typically be the edge portions of the lateral end facet of a lens, to produce a light-shielding film there has been and is being widely utilized (see, for example, Japanese Patent Application Laid-Open No. 2014-178502). Additionally, Japanese Patent Application Laid-Open No. 2008-276059 discloses a technique of reducing reflections of light by forming a light-shielding film that is opaque relative to the use wavelength of light on a textured structure less than the use wavelength of light. Such an arrangement can further reduce reflections if compared with an instance where a textured structure is formed alone in the optically non-effective area of an optical member.
Light-shielding films such as those disclosed in the above-identified patent literatures are required not only to reduce reflections of light and prevent harmful rays of light from striking the image plane but also to take a role of up-grading the appearance. In other words, such films are required not only to show excellent optical characteristics from the viewpoint of reducing reflections of light but also to be provided with a uniform texture that is free from color unevenness.
A light-shielding film described in Japanese Patent Application Laid-Open No. 2014-178502 and the use of a textured structure and a light-shielding film in combination as described in Japanese Patent Application Laid-Open No. 2008-276059 provide a uniform reflectance in the optically non-effective area of an optical member. However, in reality, as the angle 7 formed by the optically effective area 2b and the optically non-effective area 3 of a lens becomes large, there arises a phenomenon where reflected light 9 of incident light 8 that is reflected at the optically effective area 2b is concentratedly irradiated onto an optically effective area-neighboring region 5 (see FIG. 1B) of the optically non-effective area 3 (to be referred to as “boundary area” hereinafter) that is located adjacent to the optically effective area 2b. Then, the quantity of light reflected from the boundary area 5 outstandingly becomes greater if compared with the quantity of light reflected from all the remaining region so that consequently light reflected from the boundary area 5 appears as a white ring-shaped color unevenness area when viewed from the light-entering side of the lens to down-grade the appearance of the lens.
In view of the above-identified problem, it is therefore the object of the present invention to provide an optical member that can effectively suppress reflections of light at the boundary area 5 to realize a high appearance grade for the optical member.